1. Field of the Invention
This invention relates to a filter, and more particularly to a filter of the type in which at least a part of a casing is buried together with its suction ports in pebbles spread inside a water tank in order to let the pebbles exhibit a filter operation, and suction means is connected to the casing.
2. Description of the Prior Art
A filter of the kind described above has been used generally as a so-called "bottom surface type" filter. The filter uses pebbles as a filter material and filtrates water by causing the water to flow from above to below through a layer of pebbles which are spread inside a water tank. In the filter of this kind, however, the filtration efficiency is low because the particle diameter of the pebbles is relatively great. Therefore, it is theoretically possible to further filtrate the water, that has once been filtrated by the pebbles, by use of a filter material to improve the filtration efficiency, but such a filter material involves the problem that it is likely to get clogged. Accordingly, it is desired to easily eliminate such clogging.